There are many industrial effluent solutions containing high levels of sulfates and dissolved metals. Often, treatment of these solutions represents a major expense to the source operations. For example, the mincing industry is particularly concerned with acid mine drainage resulting from the oxidation of sulfide-containing waste rock, tailings and exposed mine openings. In some cases, treating these effluents represents a major factor in determining the economic viability of an industrial operation. Often, these discharges remain a concern long after industrial operations have been abandoned.
Currently, one of the most come-on methods of treating solutions containing high sulfate and metal ion concentrations involves the use of time to raise the pH and precipitate metals as hydroxides. There are several drawbacks to this treatment method, the most significant of which is the need to dispose of the resulting metal hydroxide and gypsum sludge. These sludges can be very voluminous and the metals are subjects to re-dissolution if exposed to a low pH solution. In some jurisdictions, such sludge is considered to be a hazardous waste when it contains relatively low levels of toxic metals such as Cd, Sb or Hg and it must be stabilized and stored at considerable expense. It is likely that future discharge guidelines will continue to become more stringent in all jurisdictions, with a corresponding increase in the costs of treating such solutions to comply with environmental regulations.
Reduction of oxidized sulfur in solution occurs through the action of certain species of bacteria which, under anaerobic conditions, can utilize sulfate as an electron acceptor in their metabolism. The main genera of bacteria capable of sulfate reduction are Desulfovibrio and Desulfotomaculum. There are presently about 17 species identified in these two genera. Other sulfate reducing bacteria are members of the genera Desulfobacter, Desulfobacterium, Desulfobulbus, Desulfococcus, Desulfomonas, Desulfonema and Desulfurococcus.
Sulfate reducing bacteria are found in a variety of anaerobic environments in nature, such as marine sediments, wetlands, mud flats and standing water. They are also common in anaerobic digesters in sewage treatment plants.
Sulfate reducing bacteria can grow on a variety of substrates, ideally simple organic compounds and fermentation products such as lactate, pyruvate or citrate. Some species can grow autotrophically on CO.sub.2 and H.sub.2. In mixed cultures, Sulfate reducing bacteria normally exist in a symbiotic relationship with other bacterial species which can convert a wide variety of organic and inorganic substrates into compounds which can in turn be utilized by the Sulfate reducing bacteria. The nutrients which can be used to sustain sulfate reduction, therefore include organic sources such as sewage sludge, molasses and other sugars, straw and other humic acid sources, sawdust and other ground cellulose, animal matter such as fishmeal, or pure organic compounds such as organic acids and alcohols. The end products of the metabolism of sulfate reducing bacteria are either CO.sub.2 or acetate, depending on the species and the substrate.
The biochemical reactions carried out as a part of the. sulfate reduction process can be simplified into the following equations. For system utilizing an organic substrate, using lactate as an example: ##STR1##
This may be generalized to: EQU 2C+SO.sub.4.sup.2- +2H.sub.2 O.fwdarw.H.sub.2 S+2HCO.sub.3.sup.-
For autotrophic system using H.sub.2 as an energy source, the net reaction may be represented as: EQU SO.sub.4.sup.2- +4H.sub.2 .fwdarw.2H.sub.2 O+H.sub.2 S+20H.sup.-
The role of the bacteria in these reactions is to act as a catalyst for sulfate reduction through hydrogen or electron transport occurring as a part of the organism's metabolism.
Researchers studying sulfate reducing bacteria have observed a wide variation in reduction rates, depending on such factors as reactor design, substrate used, temperature, and sulfide removal efficiency.
While all Sulfate reducing bacteria are anaerobic, most species can survive exposure to oxygen. Generally, no sulfate reduction will occur when oxygen is present because the bacteria utilize O.sub.2 in preference to SO.sub.4. Sulfate reducing bacteria are also generally tolerant of pH changes, being most active in the pH range of 5.5-8.0, but surviving at pH levels well outside of this range. The optimum pH is 6.5-7.0 for most species. The optimum temperature is typically about 31.degree. C.
Sulfate reducing bacteria tend to be tolerant of moderate concentrations of metals and S.sup.= in solution. High concentrations of metal ions may, however, interfere with bacterial growth and metabolism. Very high metal concentrations are generally prevented in active sulfate reducing environments through the precipitation of metal sulfides. Very high levels of sulfide ions in solution will inhibit sulfate reducing bacteria, so that any reactor design must incorporate adequate sulfide removal.
The application of sulfate reducing bacteria to sulfate-containing wastes has been studied for many years in substantial detail (Barnes et al 1991, Dvorak et al 1991, Gyure et al 1990, Hammack et al 1993 and 1994, Maree et al 1986 and 1987, and Tuttle et al 1969). Conventional biological sulfate reduction utilizes a bioreactor where sulfate reducing bacteria grow on some form of solid support or in a sludge bed. Sulfate is metabolized according to the equation shown below: EQU SO4+Nutrients+H.sub.2 O.fwdarw.H.sub.2 S+HCO.sub.3.sup.-
The biogenic H.sub.2 S produced by sulfate reducing bacteria may be used for metal sulfide precipitation. Metal sulfide precipitation may occur when solutions containing metal ions are contacted with H.sub.2 S gas. A general equation for this reaction shown below (where M.sup.2+ represents a metal ion having a valence of 2+): EQU M.sup.2+ +S.sup.2- .fwdarw.MS
Commonly, the solution to be treated will be at a low pH, meaning that the solubility of S.sup.2- will be low. In order to effectively remove all metals from solution, H.sub.2 S solubility must be increased by raising the solution pH or by increasing the partial pressure of H.sub.2 S in the system, since the solubility of S.sup.2- increases substantially with relatively small increases in pressure. Catalysts may also be beneficial for increasing reaction rates. At low pH values, at which S.sup.2- solubility is also low, Hg, Ag, Cu and Bi are readily removed and Fe.sup.3+ is reduced to Fe.sup.2+ by oxidation of S.sup.2-. At elevated pH values and higher S.sup.2- concentrations, Cd, Pb and Zn are easily removed. Co, Ni, Fe, Mn and other less common metals can also be removed with the proper conditions. As.sup.3+ is also precipitated at low pH as As.sub.2 S, which is slightly soluble so that a residual As concentration will remain in solution.
The solubilities of these sulfides are invariably lower than those of the corresponding metal hydroxides and they will be less likely to redissolve if the pH of the solution should decrease.
In addition to metal sulfides, biogenic H.sub.2 S may be used to form elemental sulfur by any one of several means, the most common being that of the Claus process shown below: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S.sup.0 +2H.sub.2 O,
or through the use of Fe.sup.3+ in solution: EQU 2Fe.sup.3+ +S.sup.2- .fwdarw.S.sup.0 +2Fe.sup.2+.
Biogenic H.sub.2 S may also be used to form other potentially valuable compounds; for example, NaHS: EQU 2H.sub.2 S+Na2CO.sub.3 .fwdarw.2NaHS+H.sub.2 CO.sub.3.
Many prior art sulfate reduction processes treat the entire stream in a bioreactor. This gives rise to significant limitations in terms of how such processes may be applied and in their effectiveness. Typically, the sensitivity of the bacterial population to low pH and high metal loading necessitates prohibitively long retention times for the treatment of highly contaminated streams. In addition, because the entire solution enters biological treatment, the bioreactor is typically subjected to widely varying conditions of flow and feed stream strength, with seasonal fluctuations, making it difficult to maintain the chemostat conditions necessary for optimum bioreactor performance.
The nature of the sludge produced by many prior art sulfate reduction processes gives rise to a variety of problems. If sulfide sludge is precipitated in the bioreactor, it may cause problems of plugging, abrasion, and toxicity. For example, a large volume of sludge may be contaminated by a single toxic metal sulfide, increasing the overall expense associated with sludge disposal. The sludge may also contain biomass (lost from the bioreactor), which further increases the volume of sludge for disposal, raising disposal costs. Conventional sulfate reduction processes are, however, well suited to certain specific applications, particularly those concerned with treatment of stream with low metal ion concentrations (Barnes et al, 1992), and those requiring a complete removal of sulfate.
Prior art processes in the field of sulfate reduction use either prohibitively expensive nutrients such as ethanol or lactate, or microbially challenging nutrients such as compost or other organic waste, thereby necessitating prohibitively large reactors and/or reaction times. In either case, the economic viability of the process suffers substantially.
Partial oxidation burners have been used extensively in the petroleum and chemical industries for a number of years. Partial oxidation burners are typically used to produce large quantities of extremely pure hydrogen. These prior art burners are generally orders of magnitude too large for use in providing feedstock to a microbiological culture. Prior art partial oxidation burners are generally designed to produce gases to a higher standard of purity than is required for microbiological feedstock. Such burners typically consume pure oxygen and operate at high pressures (400-3000 psi) and temperatures (2000.degree. F., for example), adding to their expense.
In a partial oxidation burner, a hydrocarbon fuel is oxidized in an oxygen-limited environment in which oxidation of the hydrocarbon is halted at the production of CO and H.sub.2, rather than proceeding to complete oxidation of the hydrocarbon to CO.sub.2 and H.sub.2 O. A second stage may be included in the partial oxidation burner to convert H.sub.2 O and CO to H.sub.2 and CO.sub.2.
In the first stage of the partial oxidation process, a. hydrocarbon, such as methane is reacted with a limited quantity of air, as follows: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2
The oxidation of the hydrocarbon feedstock is an exothermic reaction such provides heat for the second stage of the partial oxidation reaction and also provides a substantial amount of excess heat.
The first stage reaction can occur at ambient pressure, or at much higher pressures. The reaction must be catalyzed if the reactor temperature is less than about 1300.degree. K. Higher temperatures are also required if longer-chain hydrocarbon feedstocks are used (eg. &gt;C4), to reduce `cracking` and maintain the desired hydrogen-producing reaction.
In the second stage of the partial oxidation process, the gas mix resulting from the first-stage partial oxidation reaction is taken directly into a second chamber. In the second chamber, CO is reacted with water (steam) in the presence of a catalyst (usually NiO) to produce additional H.sub.2, as follow: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
The second, stage reaction is endothermic and consumes some of the heat generated in the first stage.